The National Provincial Championship (NPC) was started in 1976. It formalised the playing of provincial rugby in New Zealand, which until that time had a lot of interprovincial rivalries but no structure. It has changed it's format several times since then and also had name changes as sponsors have changed.
In 1976 there was a Division 1 made up of 7 North Island teams and 4 South Island teams. There were 2 Division 2s, one for North Island teams and one for South Island teams. The lowest placed South Island team in Division 1 played off against the winner of the South Island Division 2. This ensured that there was always 4 South Island teams in Division 1. The lowest placed North Island team was relegated, replaced by the winner of the North Island Division 2.
NB: this created an unfair situation as Taranaki found out in 1979 when they were relegated despite finishing above 3 South Island teams.
Following this changes were made in 1980. The Division 2(NI) winner played off against the Division 2(SI) winner and then the winner of that match played off against the bottom Division 1 team. This arrangement meant that there was no longer always 4 South Island teams in Division 1.
In 1985 changes were made with the creation of a Division 3. Divisions 1 and 2 were made up of North and South Island teams combined. Division 3 was split between North and South Islands the same as Division 2 was previously. The bottom placed team in Divisions 1 and 2 were relegated. The winner of Division 2 was promoted to Division 1. The winners of the two Division 3 competitions played off to determine who would move up to Division 2.
In 1992 the format was changed to 3 Divisions of 9 teams each. Since then there have been semi-finals and a final in each division.
In 1998 Division 1 was increased to 10 teams, leaving 9 teams in Division 2 and 8 in Division 3. This meant that there was no bye required in Division 1. Automatic promotion/relegation was also ended. Teams would play off to determine where they played the following year in Divisions 1 & 2.
In 2001 the numbers of teams in each division was 11, 9 & 7, in 2002 it was 10, 8 & 9 and in 2003 it was back to 10, 9, & 8.
This format remained until the end of the 2005 season. From 2006 the NPC morphed into the Air New Zealand Cup and Heartland Championship. In 2016 it was named the ITM Cup and then in 2021 the Bunnings NPC.